As is known in the art, one type of packet transmission network is a wireless transmission network. As is also known, Wireless Local Area Networks (WLAN) are becoming more and more popular nowadays due to their easy deployment and wide spread of WiFi interface cards. A Wi-Fi Alliance report finds that 1.2 million 802.11 chipsets will be produced in 2006. Parallel to technological development, a flurry of analytical studies appeared in communication literature. Experimental results and theoretical studies show that wireless networks may enter a saturation regime characterized by a highly suboptimal medium utilization. More specifically, standard rate adaptation mechanisms reduce transmission rates when multiple packet loss occurs. Yet if the packet loss is due to collision rather than bad channel (which is the working assumption for the rate adaptation mechanism) then the controller induces a higher probability of collision which snowballs in turn into an even lower throughput. Such a mechanism is used by the Automatic Rate Fallback (ARF) algorithm used in WLAN-II products from Lucent which assumes all packet loss are due to bad channel.
In experiments with voice over wireless LAN, saturation induces catastrophic failures of the access point and clients. Clients drop voice calls. These can occur as early as 10-12 handsets and as late as 16-18 handsets, depending on the configuration, channel quality, interference at the time of experimentation. In FIG. 3 one can see the total number of voice packets in an IEEE 802.11b experiment for a scenario involving voice calls between 16 handsets (eight calls). The sharp decrease in voice packets indicates saturation.
In accordance with the present invention, method and apparatus are provided to inspect wireless traffic parameters and, based on such input, to drop packets or influence admission control to avoid wireless saturation. In particular, the document discloses specific methods, called mitigation methods, to control or shape traffic, in conjunction with information about when the wireless channel is pre saturated. The procedure can be applied either in an access point (AP) or a client.
As described in two other co-pending patent application, one entitled, “Method for Congestion Detection in Packet Transmission Networks” filed on the same date as this application, and assigned to the same assignee as the present application, assigned Ser. No. 12/206,069, the entire subject matter thereof being incorporated herein by reference, and the other entitled “Method and Apparatus for Estimating Collision Probability in a Wireless Network”, filed on the same date as this application, and assigned to the same assignee as the present application, assigned Ser. No. 12/206,059, the entire subject matter thereof being incorporated herein by reference, two statistical methods are described for detecting the potential for wireless saturation by inspecting traffic parameters or features: The first using high level features of the radio channel accessible in the station (i.e. access point); and the second estimating directly the collision probability for a given (or all) access categories based on fine grain channel usage statistics. In the first case features of access to the radio channel are, for example, time utilization of the channel, number of retries, and delay. They can be recorded for a large variety of wireless data transmission situations (including voice over WLAN, when the interest is in voice, i.e. access category 3). A classification machine can automatically separate the regimes of saturation and normal operation. The classification logic can be incorporated for on-line, real-time operation in the firmware of a wireless station. In the second case, fine grained statistics can include the number of slots generated during a transmission, the total number of deferrals, and the total number of unsuccessful transmissions.
This document describes mitigation solutions working in conjunction with a solution of saturation prediction. Mitigation intervenes once pre-saturation is detected, in order to influence admission control or determine which packets should be disregarded (dropped) in order to alleviate saturation problems.
In accordance with the invention, a method is provided for controlling packet flow in a packet transmission network. The method includes determining a parameter representative of packet congestion on the network, and adjusting a flow of packets onto the network in accordance with such parameter.
In one embodiment, the parameter is a measure of saturation level of the channel.
In one embodiment, saturation level is a function of packet arrival rate at a receiver on the channel and total packet throughput on the channel.
In one embodiment, the function is that if there is a set of decreases in the packet arrival rates at each receiver that produces an increase in the total throughput, the channel is at the saturation level of the channel.
In one embodiment, the adjusting is a function of the time history of the parameter and the time average of such parameter relative to predetermined threshold levels.
In one embodiment, the adjusting selects one of a plurality of states, transitions between the states being a function of the time history of the parameter and the time average of such parameter relative to predetermined threshold levels.
In one embodiment, the parameter is a function of at least one of: time delay between transmission starts of a station on the channel and termination of the previously transmitted packet from such station; the fraction of time the channel is busy with transmissions, regardless of the origin of the transmission, or whether packets were correctly transmitted and received; and, average number of packet transmission retries on the channel.
In one embodiment, a method is provided for controlling packet flow in a packet transmission network. The method includes: during a training mode, generating a mathematical relationship between the degree of packet congestion on the channel and a plurality of measurable features of the network over a plurality of network conditions; during a subsequent normal operating mode, periodically measuring the plurality of measurable features and applying the generated mathematical relationship to such periodically measured plurality of measurable features to determine actual degree of congestion on the channel; and adjusting a flow of packets onto the network in accordance with such parameter.
In one embodiment, the degree of congestion is saturation level of the channel.
In one embodiment, the parameter is a function of at least one of: time delay between transmission starts of a station on the channel and termination of the previously transmitted packet from such station; the fraction of time the channel is busy with transmissions, regardless of the origin of the transmission, or whether packets were correctly transmitted and received; and, average number of packet transmission retries on the channel.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
Like reference symbols in the various drawings indicate like elements.